Regional Variability in Frost Penetration Depth and Frost Susceptibility of Pavement Subgrade Soils in Kazakhstan

This study evaluates regional variability in seasonal frost-penetration depth beneath the existing pavement structure and the frost susceptibility of pavement subgrade soils under the climatic conditions of Kazakhstan. Normative frost-depth values were compared with values calculated using a freezing index-based expression for eight cities representing northern, southern, western, and eastern Kazakhstan. Relative differences between the two estimates ranged from 6.6% to 37.7% (mean absolute percentage difference: 23.2%). The formula-based estimate exceeded the normative value in seven cities, whereas it was 37.7% lower in Shymkent, demonstrating that a uniform calculation approach may introduce substantial regional bias. Complementary laboratory tests performed in accordance with GOST 28622 on seven soil specimens produced final vertical heave values of 7.16–18.91 mm. Lean clay, clayey sand, and silty sand generally showed greater heave than the tested poorly graded sand soils, although one clayey-sand specimen exhibited a markedly lower response, highlighting within-class variability and the need to account for moisture state and fines characteristics. The laboratory measurements provide a controlled assessment of relative frost susceptibility; they are not a direct validation of regional frost-depth estimates. The findings support region-specific calibration of frost-depth calculations and the combined use of drainage, non-frost-susceptible replacement layers, thermal insulation, and geosynthetic reinforcement where local hydrogeological and traffic conditions justify these measures.